1. Field of the Invention
The invention relates generally to a method of determining the fatigue of structures, and more particularly to a method of forecasting the estimated fatigue damage of a solid propellant rocket motor before and after ignition, based on dynamic and historical parameters including age, thermal contraction, conditioning, pressure, temperature at ignition, and the stress free temperature of the propellant.
2. Background
Grain structural integrity of the solid propellant of rocket motors can be the limiting factor for the usable service life. Structural failure can result in catastrophic motor failure. In the case of solid propellant rocket motors, one of the most common causes for failure is stress arising from thermal contraction. The modeling of stress is complicated as the propellant changes to relieve some of this stress. These changes generally consist of viscous flow of the polymeric binder and changes in crosslinking. As the propellant ages, in general, there is a shift in the stress-free temperature toward a lower temperature.
The level of stress within the motor must be known in order to estimate damage. This level is usually determined by performing a finite element analysis. For complicated structures the corresponding finite element model may be quite large, requiring a significant amount of computation. If the load sequence is long and varied it may be necessary to make many runs of the finite element model to compute the corresponding stress sequence. For moderate to large finite element models, the time needed is often so large that it is not practical to perform the calculation. Rather, engineering judgments and approximations are sometimes made, so that many of the loads suspected of causing little or no damage are ignored. This approach is at best an imprecise process.
U.S. Pat. No. 6,301,970 to Biggs et al. teaches a method of predicting fatigue failure in a filled polymeric material. The method involves the calculation of stress at the region of highest stress using an equation which includes as parameters, regression coefficients of the stress versus modulus obtained from a finite element analysis. Once the regression coefficients are obtained, there is no further need to perform a finite element analysis. The calculated stresses are numerically integrated in a damage equation using a Monte Carlo method. The model provides an estimate of when failure will occur. The Biggs method has been tested in the case of temperature stress loading of a solid propellant rocket motor. The method does not address the effect of ignition, where high pressure and high temperature combustion gases are produced in a very short period of time. Also, the stress model is not specific as to the probable effect on the stress at the bore of the rocket motor.